Anodic brightening and polishing of metal



United States Patent Ofi ice 3,068,158 Patented Dec. 11, 1962 i3,068,158 ANODIC BRIGHTENING AND POLISHING F METAL Heinz Griiss,Berlin-Wannsee, Germany, assignor to Siemens-SchuckertwerkeAktiengesellschaft, Berlin-Siemensstadt, Germany, a corporation ofGermany No Drawing. Filed Jan. 20, 1959, Ser. No. 787,813 Claimspriority, application Germany Jan. 30, 1958 2 Claims. (Cl. 204-1405) Myinvention relates to the brightening and polishing of metal by anelectrolytic method.

Steel and iron have thus processed in anodic baths mainly consisting ofperchloric acid. Perchloric acid, under certain conditions, is extremelyexplosive and hence is applicable only with severe safety precautions.It is an object of my invention to eliminate such danger withoutforegoing the production of an excellent brightening or polishingeifect.

It is also known from US. Patent 2,542,779, to anodically brighten orpolish metal parts by means of a watercontaining mixture of sulphuricacid and fluoboric acid. Such mixtures are susceptible to being impairedand destroyed by cathodic and anodic current densities above rathermoderate limits. It is therefore another object of my invention toprovide for excellent brightening or polishing without entailing suchsusceptibility to trouble.

According to my invention, I use for the anodic brightening or polishingof metal, such as steel, an etching bath consisting essentially offiuoboric acid and concentrated phosphoric acid in water-free form. Theuse of such an acid mixture according to the invention eliminates theexplosion danger of the conventional perchloric acid tures and alsoexcels them in brightening effect. In comparison with the knownwater-containing solution of fluoboric acid and sulphuric acid, ananodic bath according to the invention is not damaged by high cathodicand anodic current densities.

I have further found, according to another feature of my invention, thatsimilar advantages are achieved by partially or wholly substituting theconcentrated phosphoric acid of the bath by concentrated non-aqueousacetic acid.

The brightening eifect of the electrolyte according to t e invention isnot appreciably dependent upon temperature and is obtained within a widerange of current densities. Good results are secured particularly whenoperating at normal room temperature (20 C.) with anodic currentdensities of 40 amps. per cm.

The preparation of water-free solutions of fluoboric acid and phosphoricacid and/ or acetic acid is preferably carried out with the aid ofwater-free barium borofluorate. The fluorate is produced in conventionalmanner and has been found readily soluble in concentrated phosphoricand/ or acetic acids. Preferably, a saturated solution of bariumborofluoride in concentrated phosphortic acid is used. The effect isfurther improved if, prior to using the solution, the barium ions arecaused to precipitate by adding concentrated sulphuric acid to the bathin stoichiometric quantity relative to the barium content of the bath.The resulting barium sulphate is practically completely insoluble in theresulting acid mixture. A slight excess of sulphuric acid does notimpair the polishing action of the bath.

I claim:

1. A method of brightening steel and iron metal, comprising subjectingthe metal to anodic etching in a waterfree electrolyte consisting of awater-free saturated solution of barium borofluoride in water-freephosphoric acid.

2. A method of brightening steel and iron metal, com prising subjectingthe metal to anodic etching in an essentially water-free electrolyteconsisting of a water-free solution of fiuoboric acid in water-freephosphoric acid, said solution being prepared by dissolving bariumborofluoride in phosphoric acid to saturation of the latter andprecipitating the barium as barium sulfate by adding a stoichiometricamount of sulfuric acid.

References Cited in the file of this patent UNITED STATES PATENTS2,108,603 Mason Feb. 15, 1938 2,521,106 Wiesner Sept. 5, 1950 2,920,023Bohne et a1. Jan. 5, 1960 2,928,777 Smith Mar. 15, 1960 2,986,499Wernlund May 30, 1961 FOREIGN PATENTS 550,175 Great Britain Dec. 28,1942 618,120 Great Britain Feb. 16, 1949 630,854 Great Britain Oct. 24,1949

1. A METHOD OF BRIGHTENING STELL AND IRON METAL, COMPRISING SUBJECTINGTHE METAL TO ANODIC ETCHING IN A WATERFREE ELECTROLYTE CONSISTING OF AWATER-FREE SATURATED SOLUTION OF BARIUM BOROFLUORIDE IN WATER-FREEPHOSPHORIC ACID.